Mermaids were the UFOs or aliens of Victorian times, says Harriet Ritvo, who has reeled in a collection of curious tales from an era with a mania for classifying and cataloguing animals. Tim Cornwell reports

Harriet Ritvo has seen mermaids all over the world. The Peabody Museum at Harvard university in the United States has two, tucked away in a drawer. One takes pride of place at the little-known Horniman Museum in South London. "They are small and ugly and shrivelled up," she says. "They are a completely different species of thing from the Little Mermaid-type of mermaid."

A number of these stuffed mermaids circulated in the West until well into the 19th century. Triumphs of taxidermy, most are thought to have been manufactured in Japan for the Indonesian market, for reasons that are still obscure. In 1822, a Captain Eades sold his ship to pay for one and brought it back to Britain.

At Eades's request William Clift, a curator of the Hunterian collection at the Royal College of Surgeons in London, arrived to examine it. He instantly pronounced it a fraud, consisting of the cobbled-together remains of an orang-utan, a baboon and a salmon, with synthetic nails and shortened arms. Others were made in the same way, some fitted with human teeth.

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Clift's etchings and a more recent photo of the mermaid appear in Ritvo's new book, The Platypus and the Mermaid, and Other Figments of the Classifying Imagination. Ritvo calls it a study of "animal classification, construed in the largest possible sense". In their bid to harness the unknown, Victorians had a mania for classifying and cataloguing, and from this potentially dry subject Ritvo segues into delicious curiosities. The book includes, for example, a set of meticulously drawn portraits showing the racial distinctions between specimens of Britons from Lincolnshire, Nottinghamshire, Yorkshire and Sussex (the man from Sussex was clean-shaven).

"It is not exactly a history-of-science book," she says. Instead, she takes the well-worn anthropological observation that the classification of animals tells as much about the people who classify them as it does about the classified and works from there. The book dwells heavily on the Victorians, but spans a time when the standards of scientific knowledge moved out from medieval-style bestiaries through Charles Darwin to classifying species at the rate of thousands a year.

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Ritvo, a professor of history at the Massachusetts Institute of Technology, was trained in Victorian literature but moved into Victorian history. The book developed from her previous one, The Animal Estate: The English and Other Creatures in The Victorian Age. She has made the cultural history of animals her speciality, partly inspired by a favourite uncle who was a veterinarian. "It is a wonderful subject," she says. "Animals are very important culturally, economically and emotionally. When people talk about animals, they kind of let it all hang out. They are unselfconscious in ways that they are not when discussing people."

The book looks at how contemporary prejudices about race or sex shaped the way people classified animals, of human and other varieties. It chronicles misconceptions that were revealing as well as wrong. One of the ways people can distort the past, Ritvo observes, is by looking only at the antecedents of what we now know to be true. "There is also all this other stuff that has been discredited, but which people did not know to discredit then."

Along with mermaids, Victorian animal husbandry included a lot of bizarre ideas about reproduction, which seemed to borrow heavily from Victorian attitudes to women. One preconception - that genetic characteristics were inherited almost wholly by the male, with the female as the "fertile soil" - withered in the face of the obvious inheritance of traits by female animals. But the literature of breeding continued to take a "boys will be boys attitude" to male animals, says Ritvo, with only the proviso being that scattering their sperm too liberally would weaken their offspring.

Ritvo's researches took her to the archives of both the Kennel Club and the National Horse Racing Museum at Newmarket. She found a persistent worry that "if a female cow becomes smitten with a bull of the wrong breed,even if you mate her with her own breed she is going to produce a calf that looks like the object of her affection, a kind of illegitimate power that females have over their offspring". The idea that the first male who fathers a child of a female contaminates her, even if he never sees her again, was mirrored in the old wives' tale that if a man married a widow, his children would be bastards - in some way children of his wife's first husband as well. All kinds of breeding manuals, especially those for dogs and cats, warned that if a female produced a mongrel litter from some unauthorised liaison, the pedigree of succeeding litters would not be guaranteed, even with a pedigree sire. This palpable nonsense was almost universally accepted by 19th-century breeders, and even led Darwin astray. He cited the famous example of Lord Morton's Mare, which had her first foal by a quagga, a striped relative of the zebra that became extinct in the 1870s. The mare's subsequent foals were observed to have faint striping.

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Mermaids betrayed a different human instinct. They were the UFOs or aliens of their era, Ritvo points out: strange creatures that defy logic but that we persist in believing are Out There, or even Over Here. Mermaid tales crop up repeatedly in Victorian history, as do stories of UFOs after 1945 - UFOs linked to the early exploration of space, mermaids to that of the sea. Nearly half of all Americans, according to polls, believe not just that aliens exist but that they have visited us. Between 1947 and 1969, the Pentagon investigated 12,618 UFO sightings.

Scientists in the early 19th century were enlisting in the business of busting mermaid fakes, pointing out the likely confusion between mermaids and the dugong, or sea cow. Likewise, in the late 20th century, they have felt the need to challenge contemporary myths of alien communications in films such as Contact, or Jurassic Park's tale of dinosaurs recreated from DNA in amber.

In the tiny town of Rachel, Nevada, on the road officially rechristened the Extraterrestrial Highway, local UFO enthusiasts display the remnants of strangely mutilated cows as evidence of alien landings. "Corpses" from a crashed spacecraft feature in videos of the "alien autopsy", a favourite of UFO fans, though the military now suggests that these eerie figures were probably inspired by crash dummies. There is a powerful parallel between these modern mermaids and the shrivelled and unsightly figures that Ritvo encountered at Harvard's Peabody Museum.